1. Field of the Invention
This invention relates to a voltage detection circuit, and more particularly to a voltage detection circuit formed of an integrated circuit of insulated gate type field effect transistors (hereinbelow, referred to as MIS FETs).
2. Description of the Prior Art
It is necessary for a battery-powered electronic device, e.g., electronic wristwatch, to indicate the time of battery exchange by some means after the consumption of a predetermined capacity of the battery since a battery is sooner or later used up. Such conventional devices are described in for example, Japanese Laid-Open Patent Publications No. 49-71968 dated July 11, 1974 (Japanese Patent Application No. 47-112465) and No. 51-13280 dated Feb. 2, 1976 (Japanese Patent Application No. 49-80932). The consumption of the battery capacity appears as a decrease in the electromotive force. Hence, the time for battery exchange can be detected by monitoring the decrease in the electromotive force of the battery.
In this case, especially for an electronic wristwatch, it is preferable to integrate a voltage detection circuit in a monolithic semiconductor integrated circuit (IC) from the points of cost performance and packaging space. The voltage detection circuit can be basically formed of series resistances R.sub.A and R.sub.B for dividing the source voltage V.sub.DD from a battery source such as mercury, silver or lithium battery and an inverter INV receiving this divided voltage as the input, as shown in FIG. 6. In an IC chip including a semiconductor integrated circuit, however, resistive elements formed of MIS structure resistors or diffused resistors have a relatively large dispersion of resistances, and the dispersion of the threshold voltage of the MIS FETs constituting the inverter in the IC chip and the temperature variation of the threshold voltage of the complete circuit are relatively large. Therefore, the above circuit cannot be directly used as the voltage detection circuit in an IC. For example, the electromotive force of the silver battery of a rating voltage 1.5 volt used in the electronic wristwatch, etc. gradually falls down to about 1.4 volt and then rapidly falls down, while the assured minimum operative voltage for the electronic circuit is 1.25 volt. Thus, the voltage detection circuit should detect the voltage of around 1.32 volt with an accuracy not lower than .+-.0.075 V.